1. Field of the Invention
The present invention relates generally to baseband signal generators for digital modulators, and more particularly, to a baseband signal generator for a digital modulator used as a MODEM for a digital communication equipment such as a land mobile radio telephone, a portable radio telephone and a cordless telephone.
2. Description of the Background Art
A conventional digital communication apparatus modulates a carrier signal in response to a digital information signal (baseband signal) to transmit the information signal in order to achieve efficient transmission.
Such modulation systems include an amplitude modulation system wherein an amplitude of a carrier signal is changed in response to a digital baseband signal (a modulating wave signal), a frequency modulating system wherein a frequency of a carrier is deviated in response to a modulating wave signal, a phase modulating system wherein a phase of a carrier is changed in response to a modulating wave signal and an amplitude phase modulating system wherein an amplitude and a phase of a carrier are individually changed in response to a modulating wave signal.
The carrier signal (modulated signal) S(t) thus modulated in response to a modulating wave signal can be generally expressed by the following equation. ##EQU1##
Herein, A(t) denotes an amplitude, .omega..sub.c denotes a carrier frequency and .phi.(t) denotes a phase of a modulating wave signal.
As is clear from the above-described equation (1), the modulated signal can be represented by two components orthogonal to each other, that is, by a sum of an in-phase (I phase) component (the first term of the above-described equation (1)) and a quadrature phase (Q phase) component (the second term of the above-described equation (1)). Such a modulated signal can be therefore formed by using a quadrature modulator.
FIGS. 1 and 2 are a block diagram and a spatial diagram schematically showing the principle of such a quadrature modulator, respectively. It should be noted that the following example shows a phase modulating system for changing a phase of a carrier in response to a baseband signal, wherein an amplitude A (t) is fixed to "1".
With reference to FIG. 1, a mapping circuit 2 outputs I phase and Q phase components of a modulating wave signal as rectangular signals in response to a digital baseband signal applied through an input terminal 1. The I phase component is applied to one input of a multiplier 7 through a low pass filter (LPF) 3, while the Q phase component is applied to one input of a multiplier 8 through a low pass filter LPF 4.
A carrier signal cos.omega..sub.c t is applied from a signal source 5 to the other input of the multiplier 7 which outputs an I phase component sin.phi.(t).multidot.cos.omega..sub.c t of a modulated signal. A signal sin.omega..sub.c t obtained by shifting the phase of the carrier signal from the signal source 5 by .pi./2 by means of a phase shift circuit 6 is applied to the other input of the multiplier 8 which outputs an Q phase component cos.phi.(t).multidot.sin.omega..sub.c t of the modulated signal. Thus obtained I phase component and Q phase component can be represented in a one-to-one correspondence on the I and Q coordinates as shown in FIG. 2.
These I phase component and Q phase component are added to each other by an adder 9 to become such a modulated signal as expressed by equation (1), which signal is output from an output terminal 10.
The above described mapping circuit 2 includes two ROMs wherein signal waveform data, which have been obtained in advance by calculation, of I and Q phases of the digital modulating wave signal with their bandwidths being limited are stored, respectively. Such waveform data are read out from the ROMs using the digital baseband signal applied through the input terminal 1 as addresses.
There is a case where M-phase PSK (Phase Shift Keying) signal is generated by using such a quadrature modulator. FIG. 3 is a diagram schematically showing the principle of the generation of .pi./4 shift QPSK (Quadli Phase Shift Keying) signal, which signal is one example of such a M-phase PSK signal.
With reference to FIG. 3, it is assumed that a signal point corresponding to I phase component and Q phase component data of a baseband signal (modulating wave signal) at a certain time point exists at one of a, c, e and g on the unit circle having a radius of 1 shown in FIG. 3. At a subsequent time point after a lapse of a predetermined time slot, the signal point shifts to one of the intersections b, d, f and h between two virtual axis obtained by rotating their axis and the Q axis by .pi./4 and the unit circle of a radius of 1. The I axis and the Q axis will be rotated by .pi./4 for each predetermined time slot in the same manner as described above, whereby the signal point sequentially shifts on the unit circle.
For example, if the signal point initially exists at the point a in FIG. 3 and the baseband signal does not change, the signal point shifts as a point .fwdarw.b point.fwdarw.c point.fwdarw.d point.fwdarw.e point.fwdarw.f point.fwdarw.g point.fwdarw.h point for every predetermined time slot, that is, at every .pi./4 rotation of the I axis and the Q axis. In this case, the I and Q phase data each takes the five types of values such as "1", "1/.sqroot.2", "0", "-1/.sqroot.2" and "-1" as can be seen from FIG. 4.
On the other hand, according to the digital cellular telecommunication system standard of Japan (RCR) and the cellular telecommunication standard (TIA-IS-54) of the North America, differential encodings are carried out in .pi./4 shift QPSK modulation. Because of such differential encoding, it is only necessary to consider a relative phase between continuous symbols. Therefore, by shifting the phase of the signal spatial diagram of FIG. 3 by .pi./8 as shown in FIG. 4, data of the I phase and the Q phase have levels of four value. Such .pi./4 shift QPSK modulation by such differential encoding is generally referred to as ".pi./4 shift DQPSK modulation".
Brief description will be given to a procedure of generating a baseband signal for such .pi./4 shift DQPSK modulation. First, an applied serial digital baseband signal is converted into symbol data of (X.sub.k, Y.sub.k) by a serial/parallel conversion circuit. Then, the symbol data (X.sub.k, Y.sub.k) is converted into a quadrature signal (I.sub.k, Q.sub.k) based on the following equation by a differential encoding and mapping circuit. ##EQU2##
Herein, the above-described .DELTA..phi. (X.sub.k, Y.sub.k) is defined based on the following table.
______________________________________ X.sub.k 1 0 0 1 Y.sub.k 1 1 0 0 ______________________________________ .DELTA..phi. -3.pi./4 3.pi./4 .pi./4 -.pi./4 ______________________________________
Thus obtained signals I.sub.k and Q.sub.k are band-limited by low pass filters and then supplied to a quadrature modulator as an I phase component and a Q phase component of the baseband signal, respectively.
FIG. 5 is a diagram schematically showing a structure of a baseband signal generator which uses such principle and is the background art of the present invention. With reference to FIG. 5, a serial digital baseband signal applied through an input terminal 21 is converted into parallel 2-bit data by a serial/parallel conversion circuit 22. A differential encoding and mapping circuit 23 differentially encodes the current 2-bit data from the serial/parallel conversion circuit 22 and 2-bit data of an immediately preceding clock, while performing mapping on a signal spatial diagram. A timing signal generation circuit 24 is driven by a clock signal having a frequency higher than that of a symbol rate to generate a clock signal for an input signal, a clock signal having a symbol period and clock signals for digital filters.
Symbol mapping data of the I phase and the Q phase output from the differential encoding and mapping circuit 23 are band-limited by digital filters 25 and 26 each having impulse response characteristics of a route Nyquist filter and applied to D/A converters 27 and 28, respectively. As a result, the D/A converters 27 and 28 respectively provide band-limited analog baseband signals of the I phase and the Q phase, which signals are applied to a modulated signal generation unit 31 (corresponding to the elements 5 to 9 of FIG. 1) through output terminals 29 and 30. Then, a generated modulated signal is output through an output terminal 32.
FIG. 6 is a block diagram showing a structure of one digital filter 25 shown in FIG. 6. The other digital filter 26 is also structured similarly to the digital filter 25. The digital filter 25 includes a symbol mapping data accumulation circuit 25a for shifting the symbol mapping data of the I phase supplied from the differential encoding and mapping circuit 23 in response to a clock signal SCK having a symbol period supplied from the timing signal generation circuit 24 and accumulating the shifted data, and an ROM25b for storing a previously calculated waveform obtained by overlapping route Nyquist filter outputs (impulse response signals) as shown in FIG. 7. In FIG. 6, 2 bits of M51 and M50 correspond to the latest symbol mapping data, while 2 bits of P51 and P50 correspond to the oldest symbol mapping data.
The contents stored in the ROM25b are read out based on the symbol mapping data output from the accumulation circuit 25a and 2 bits of time information A1 and A0 supplied from the timing signal generation circuit 24. In an example which will be described in the following, an ROM forming a digital filter of each phase accumulates data corresponding to 5 symbol sections preceding to a center symbol section and following 5 symbol sections, that is, the total of 11 symbol sections (11, as the number of taps of ROM). That is, since the symbol mapping data of each phase has the levels of four values because of the above-described differential encoding, a waveform obtained by overlapping impulse response characteristics corresponding to 4.sup.11 data patterns is stored in advance in the ROM25b of FIG. 6.
An address of such ROM25b requires a total of 24 bits (=22+2) including 22 bits (=11.times.2) for accumulating the four-value symbol mapping data for 11 symbol sections and 2 bits of time information for data reading. Herein, with a data length of 8 bits, the capacity of the ROM25b is equivalent to 2.sup.27 bits (=2.sup.(22+2) .times.8). For the I phase and the Q phase in total, such a large ROM capacity as of 268 gigabits (=2.sup.27 .times.2=2.sup.28) is required.
On the other hand, as described in "An Experimental TDMA Transmission System Using 1.5 GHz-Band .pi./4 Shifted QPSK", ELECTRONIC INFORMATION COMMUNICATION CONFERENCE PROCEEDINGS B-II Vol. J73-B-II No. 11, November 1990, pp. 639-650, it is also possible to allot one ROM for each level of the data, whereby a sum of the outputs of the respective ROMs is taken. In this case, however, 13 bits (=11+2) are required for an address of each ROM. With a data length of 8 bits, the capacity of the ROM in each level is equivalent to 2.sup.16 bits (=2.sup.(11+2) .times.8). Therefore, in a case of data having levels of four values, each of the I phase and the Q phase requires 2.sup.18 bits (=2.sup.16 .times.4). So large a capacity as 2.sup.18 .times.2=2.sup.19 bits=524 kilobits is required for the I phase and the Q phase. As described in the foregoing, the larger a capacity of an ROM or the number of ROMs is required, the more difficult it is to make a modulator as an LSI while decreasing its manufacturing cost.
In a conventional digital modulator, on the other hand, it was not considered how to cope with burst transmission. Burst transmission without considering any countermeasure results in generation of transmission spurious.
More specifically, in the normal burst transmission, the data transmission in effected intermittently as shown in FIG. 8A(a). As shown therein, if the time width of transmission is defined as T.sub.B (sec), the spectrum as expressed by the following equation is generated. ##EQU3##
FIG. 8B is a graph showing such spectrum wherein the spurious transmission is caused by the portion indicated with hatching.
In order to prevent generation of such transmission spurious, a generally-called ramp processing for smooth rise and fall of burst as shown in FIG. 8A(b) is required.
FIG. 8A(c) is an enlarged diagram showing transmission waveform in such rising edge and falling edges. The following function is used as a function for the rising. ##EQU4##
The following function is used as a function for the falling edge. ##EQU5##
In the above equations (4) and (5), "T.sub.S " indicates the symbol period.
However, a conventional system needs an additional ROM dedicated for such a ramp processing. Hence, a digital modulator applicable to burst transmission by a conventional system inevitably requires an ROM having increasingly larger capacity.